Extrusion dies

Design of Extrusion Dies
Milivoje M. Kostic
Department of Mechanical Engineering, Northern Illinois University, DeKalb, Illinois, U.S.A.

Louis G. Reifschneider
Department of Technology, Illinois State University, Normal, Illinois, U.S.A.

INTRODUCTION The goal of this chapter is to introduce the reader to the importance of extrusion die design as well as the complexities inherent in thetask. Extrusion is of vital importance to all plastics processing. In addition to providing raw stock such as sheet for thermoforming and pellets for injection molding and other extrusion processing, numerous end-use products are made with extrusion such as film, tubing, and a variety of profiles. Although the types of extruded products made can differ dramatically in shape, there are a set of commonrules that govern basic die design. For example, it is important to streamline the flow from the inlet to the exit, and as a practical measure, to fine-tune the flow balance and product dimensions, flow adjustment devices could be included in the die design. Several unique products are made by extrusion and the dies needed to make these products are classified as: 1) sheet dies; 2) flat-film andblown-film dies; 3) pipe and tubing dies; 4) profile extrusion dies; and 5) co-extrusion dies. Furthermore, each product type has unique hardware downstream of the die to shape and cool the extruded melt. To aid the reader, detailed illustrations of the various die designs and the complementary downstream cooling and shaping hardware are shown. Predicting the required die profile to achieve the desiredproduct dimensions is a very complex task and requires detailed knowledge of material characteristics and flow and heat transfer phenomena, and extensive experience with extrusion processing. Extrusion die design is still more an art than a science, even though the latter is becoming more and more relevant for design optimization because of recent advancement in the powerful computation and modeling ofcomplex flow and heat transfer processes, before, through, and after the die.

and heated as they are conveyed through either a single- or a twin-screw extruder (as described elsewhere) to become a pressurized melt. The pressurized melt flows through a properly shaped orifice, or extrusion die, and then is pulled (with a little pressure) as it is cooled and shaped to a final product called theextrudate. The proper design of an extrusion die is extremely important to achieve the desired shape and accurate dimensions of the extruded product. The function of an extrusion die is to shape the molten plastic exiting an extruder into the desired cross section depending on the product being made. The die provides a passage between the circular exit of the extrusion barrel and the more complex andoften much thinner and wider die exit. A schematic of a common die, called a sheet die, is shown in Fig. 1A to illustrate this point. The extrusion process creates products of uniform cross section in a continuous fashion. An ideal passage will:[1,2]  Balance the melt flow by providing a more uniform exit velocity across the entire die exit.  Achieve this flow balance with a minimal pressure drop. Streamline the flow to avoid abrupt changes in the flow passage that may cause stagnation areas. Stagnated flow may lead to thermal degradation of the plastic melt as the melt is exposed to high heats for long periods. As a practical measure, flow control devices should be incorporated into the die design to permit finetuning of the die passage shape to ensure a proper flow balance. In addition, thedesign of extrusion dies is complicated by two unique material properties of molten plastics:[3]  Melts exhibit shear thinning behavior (become less viscous) as they are sheared.  Melts exhibit viscoelastic behavior, which influences the ‘‘die swell’’ on exiting the die.
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DESIGN FUNDAMENTALS Extrusion is a continuous process where solid polymeric materials, either pellets or powders, are...